The present invention relates to a method of manufacturing a steel ball for a rolling bearing using a pair of molding forms.
In manufacturing a steel ball for a rolling bearing, such molding forms are disposed opposing to each other. The molding forms have respectively hemispherical concave portions on the opposing faces thereof, coupling faces in peripheral edges of the concave portions, and relief grooves formed in the peripheries of the concave portions.
The method of manufacturing the steel ball will be described. A steel wire having a required wire diameter is provided. A fiber flow exists in a lengthwise direction of the steel wire. The fiber flow is constituted by a number of fiber type of metallic texture along with the lengthwise direction of the steel wire. The steel wire is cut at a constant length in accordance with the size of the steel ball to be manufactured.
The steel wire thus cut is housed in the hemispherical concave portions of the both opposing faces of the molding forms and then is molded into a spherical shape by being compressed by the concave portions. This spherically formed steel wire is referred to as a raw ball in the present specification. On the occasion of such a molding, the fiber flow on the surface of the steel wire is relieved by the relief grooves and bent outward. Thereby an annular protrusion or band portion constituted by the fiber flow bent outward is formed on the surface of the raw ball removed from the molding forms.
There is a step of executing a deburring operation to the surface of the raw ball. The fiber flow of the band portion is synchronously cut by the deburring operation in some cases. A strength of the band portion reduces when the fiber flow thereof has been cut. The steel ball manufactured from such a raw ball has an inferior durability as a rolling element of a rolling bearing. Moreover, since the band portion from the surface of the steel ball protrudes, the band portion easily collides with a raceway surface of the rolling bearing so as to easily generate a noise. Thus it is preferable to remove the band portion from the surface of the raw ball without cutting the fiber flow in consideration of manufacturing cost efficiency.
Therefore, a main object of the present invention is to provide a method of manufacturing a steel ball for a rolling bearing which can remove a band portion from a surface of a raw ball without cutting a fiber flow on a low manufacturing cost, thereby having a high durability and a low noise level during use.
In summary, a method of manufacturing a steel ball for a rolling bearing in accordance with a first aspect of the present invention comprises: a molding step of forming a steel wire cut at a predetermined length into a raw ball having a band portion by a pair of molding forms having respectively hemispherical concave portions and relief grooves in peripheries of said concave portions on their respective opposing faces, said band portion being formed on a surface of said raw ball along said relief grooves by compressing said steel wire by said molding forms in a state in which both ends of said steel wire are sandwiched between said concave portions; and a tumbler working step of removing said band portion from the surface of the raw ball by tumbling.
According to the first aspect of the present invention, the band portion is compressed and can be prevented from protruding from the surface of the raw ball thereby. In this case, different from a deburring operation, a fiber flow is not cut. The fiber flow exists on the surface of the raw ball without being cut, which enables manufacturing a steel ball having an improved rolling durability as a rolling element of a rolling bearing from the raw ball. Also, due to the absence of the band portion, it is possible to manufacture the steel ball which does not produce a noise caused by the collision of the band portion when rolling on a raceway surface of the rolling bearing. Further, since the band portion is simply compressed, a manufacturing cost can be reduced to a reasonable level. Also, the compression of the band portion impart a compression residual stress to the surface of the raw ball. As a result, the hardness of the raw ball increases according to the compression residual stress so as to improve the rolling life of the raw ball thereby.
Moreover, since the deburring operation is synchronously executed in the tumbler working step, the conventional deburring operation can be dropped out and a further manufacturing cost reduction can be realized. Since the deburring operation is not included, the cut portion of the fiber, which is subject to a cause of a breakage such as a surface flaking and the like, can be prevented from an exposure to the surface. In case of the tumbler working step, it is unnecessary to prepare a special type of molding form, which does not generate the band portion in order for a step of removing the band portion of the raw ball. Furthermore, a large amount of a grinding cost in order to remove the entire fiber flow bent with respect to the raw ball is unnecessary, whereby the manufacturing cost of the steel ball can be reduced.
As a preferred embodiment of the first aspect of the present invention, the tumbler working step is the working operation of housing the raw ball together with a medium within a drum and rotating the drum.
As a more preferred embodiment of the first aspect of present invention, a material of the steel wire is a steel material selected from a group composed of a high-carbon chromium bearing steel, a stainless steel a case hardening steel and a high-speed tool steel.
2) A method of manufacturing a steel ball for a rolling bearing in accordance with a second aspect of the invention comprises a molding step of forming a steel wire cut at a predetermined length into a raw ball having a band portion by a pair of first molding forms having respectively hemispherical concave portions and relief grooves in peripheries of said concave portions on their respective opposing faces, said band portion being formed on a surface of said raw ball along said relief grooves by compressing said steel wire by said molding forms in a state in which both ends of said steel wire are sandwiched between said concave portions of said first molding forms; and a pressing step of pressing said band portion by compressing said raw ball by a pair of second molding forms having respectively hemispherical concave portions and coupling faces in a state in which said band portion is obliquely arranged with respect to said coupling faces of the second molding forms and the raw ball is sandwiched between said concave portions of said second molding forms.
In this case, the raw ball may be gripped and in a state the band portion is disposed perpendicularly to coupling faces of the second molding forms in the pressing step.
In accordance with the second aspect of the present invention, as a consequence that the band portion of the raw ball is removed by being pressed so as to make the surface thereof smooth, the rolling durability improves and the steel ball which does not produce a noise caused by the collision with a raceway surface of a rolling bearing. Since the band portion of the raw ball is removed in the pressing step, it is unnecessary to prepare a special type of a molding form which does not produce a band portion. Also, a great deal of a grinding cost to remove an entire bent fiber flow becomes unnecessary, and consequently, the manufacturing cost can be reduced. Furthermore, since the pressing step does not comprise the conventional deburring operation, it is possible to intend to reduce the manufacturing cost. The absence of the deburring operation can prevent the cut portion of the fiber from being exposed to the surface, which can easily be a cause of a breakage such as a surface flaking and the like.
As a preferred embodiment of the second aspect of the present invention, the pressing step is repeated plural times. The repetition of the pressing step enables the surface of the raw ball to be smoother and therefore the band portion to be removed without fail. The rolling durability of a rolling bearing further improves and a noise can certainly be prevented, whereby the manufacturing cost can be reduced.
As a more preferred embodiment of the second aspect of the present invention, the first molding forms of a molding step and the second molding forms of a pressing step are common molding forms. In this manner, because the first molding forms and the second molding forms are common ones, it is unnecessary to prepare plural types of molding forms in both of the steps. Therefore, the manufacturing step can be simplified so as to intend to reduce the manufacturing cost thereby.
3) A steel ball for a rolling bearing of the present invention is structured such that a convex band portion formed on a circumference perpendicular to a circumference connecting between said polar portions thereof have been removed.
According to the steel ball for the rolling bearing of the present invention, it is possible to prevent the surface of the steel ball from flaking in the band portions during use and the rolling durability improves thereby. Further, it is possible to prevent a noise caused by a collision of the band portion of the steel ball with a raceway surface of the rolling bearing.
Additionally, in the step of manufacturing this steel ball it is preferable not to include the deburring operation applied by cutting using a flushing and the like in order to prevent the cut portion of the fiber from being exposed to the surface, which is subject to a cause of a breakage such as a surface flaking and the like.